Six dimers of gamma delta resolvase and two copies of a 120 base pair DNA segment (res) form a specific protein-DNA complex, the synaptosome, during site-specific recombination. Resolvase dimers are bound to three similar sites within each res (sites I, II and III); however, the role of resolvase is different as these sites and DNA cleavage and ligation occurs solely at site I. This suggests that resolvase interaction at site I and sites II and III within the synaptosome differ in important ways. An important and unanswered question regarding the resolvase reaction concerns synapsis of the two crossover sites. How do the two resolvase- bound copies of site I interact with one another in the synaptic complex so as to promote strand cleavage and exchange? The X-ray crystal structure of resolvase bound to site I provides few clues regarding this interaction and, thus far, mutagenesis has failed to identify the relevant protein interface. To investigate the arrangement of resolvase at site I in the synaptic complex, targeted protein foot printing will be used as a tool for determining the protein-protein interaction of resolvase dimers bound at site I. The isolation of covalently attached resolvase bound to DNA ate site I (via serine-10-a 5' phosphate) will be used to distinguish resolvase molecules at site I from those at site II and II from free resolvase. Chemical modification of surface amino acid residues and cleavage with specific proteases will help identify solvent modification of surface amino acid residues and cleavage with specific proteases will help identify solvent accessible and buried amino acid residues of resolvase in the synaptic complex. Site-directed mutagenesis of resolvase surface residues implicated in having a role in synaptic complex macromolecular interactions will be used to confirm their importance in resolvase action in site I.